A cellulose acylate film has moderate moisture permeability and is easily processed. Among cellulose films, a cellulose acylate film has higher optical isotropy (a lower retardation value) than other polymer films. Therefore, it is common to use a cellulose acylate film in applications requiring optical isotropy, such as for a polarizing plate. On the other hand, it is demanded that optical compensation sheets (retardation films (or phase-contrast films or phase difference films)) for use, for example, in a liquid crystal display device, have optical anisotropy (high retardation value), on the contrary. Therefore, a synthetic polymer film, such as a polycarbonate film or polysulfone film, which has a high retardation value, is commonly used as the optical compensation sheet.
As described above, in the technical fields of optical materials, a synthetic polymer film is used when it is required for a polymer film to have optical anisotropy (high retardation value), and a cellulose acylate film is used when it is required for a polymer film to have optical isotropy (low retardation value), in general.
However, in these days, another-type of cellulose acylate film having a high retardation value is demanded, so that it can also be used in applications (for example, an optical compensation sheet) for which optical anisotropy is required. It is proposed that a cellulose acylate film be used as a phase-contrast film by imparting optical isotropy to the film, and technologies corresponding to this are proposed (for example, European Patent Application Publication (Laid-Open) No. 0911656A2). In the above European Patent Application Publication No. 0911656A2, an aromatic compound having at least two aromatic rings, and particularly a compound having a 1,3,5-triazine ring, is added, and orientation process is carried out, in order for a cellulose acylate film to have a high retardation value, in practice.
As measures to increase the retardation of a cellulose acylate film, known are, for example, (1) a method in which a cellulose acylate film is oriented, to increase the orientation of its molecular chain; and (2) a method in which a highly anisotropic low-molecular compound is added. For example, there is disclosed a method in which a cellulose acylate propionate film is oriented using a tenter, in JP-A-2003-73485 (“JP-A” means unexamined published Japanese patent application). Also, JP-A-2002-182215 discloses a method in which a cellulose acylate film, to which a disk-like compound is added, is oriented using a tenter. The above methods, however, have the problem that, if it is intended to obtain a high retardation value, variations in retardation and slow axis within the surface of the film are increased, and therefore the problem must be improved.
Generally, it is known that a cellulose triacetate is a high-molecular raw material that can be oriented with difficulty, and it is difficult to make the cellulose triacetate have a large birefringence. However, in the above European Patent Application Publication No. 0911656A2, additives are oriented at the same time by orientation process, thereby making it possible to increase birefringence and attaining a high retardation value. This film has such a merit that an inexpensive and thin-film liquid crystal display device can be obtained, because this film can double as a protective film of a polarizing plate.
It is essential, today, to develop a thin-layer liquid crystal cell to reduce the mass of a liquid crystal display device and production cost. The optical performance required for an optical compensation sheet is a higher Re retardation value, and it has become necessary to provide a film having a lower Rth retardation value.
However, the inventors of the present invention, having made earnest studies as to the method disclosed in European Patent Application Publication No. 0911656A2, have, as a result, clarified that this method has the problem that, if it is intended to attain a high Re value, the Rth value increases, and it is therefore difficult to attain a higher Re value and a lower Rth value, with the result that, in the case of setting the aforementioned Re retardation value and Rth retardation value individually, they cannot be set compatibly. There are, besides the above European patent publication, patent documents, for example JP-A-2001-116926, which disclose techniques concerning the optical performance of a VA phase-contrast film. In the publications, no method of making a desired Re value and a desired Rth value compatible with each other is clearly described.
Also, as mentioned above, a cellulose ester film is eventually used as an optical compensation sheet doubling as a polarizing plate protective film. Along with developments of products with high precision, there is a strong demand for a high-quality cellulose ester film.
In order to use a cellulose acetate film as an optical compensation sheet doubling as a polarizing plate protective film in a liquid crystal display, in particular, lubricity (scratch resistance) of the surface of a film is required. Specifically, when a polarizing plate is produced using a polarizing film and the above film, a treatment for saponification of the film (hydrophilic treatment), a process of adhering the polarizing film onto the film by using an adhesive, and further a conveying work(s) for performing these steps, are carried out. When the scratch resistance of the film surface is insufficient, and as a result the film surface is damaged during the above operations, a liquid crystal display into which a polarizing plate using such a film is incorporated has a fatal surface defect.
A cellulose acetate film may be obtained by a solvent film forming method comprising: casting a dope, obtained by dissolving cellulose triacetate having a bonded acetic acid amount (degree of acetylation) of 60 to 62% together with a plasticizer in a mixed solvent of methylene chloride and methanol, on a drum that is continuously rotated, or a moving band (support), and then vaporizing the solvent. Then, to improve scratch resistance, silicon dioxide, which is currently used, is dispersed in a solvent, or a mixed solution of a solvent and cellulose triacetate; the resulting dispersion is mixed with the above dope, and the resulting mixed solution is cast and dried, to thereby produce a cellulose triacetate film. By this processing, irregularities are formed on the surface of the film, to impart lubricity to the surface.
However, when silicon dioxide is used as a matting agent to improve the aforementioned lubricity, a large coagulate is generated in the film, posing the problem that the transparency of the film is deteriorated.
First, the reason a high Re/Rth ratio is not attained by the method disclosed in the European Patent Application Publication No. 0911656A2 will be explained.
In the case of considering that the thickness of a film is fixed, the retardation of the film is determined by the refractive index and amount of a raw material and the orientation state. In the method disclosed in the European Patent Application Publication No. 0911656A2, the retardation of the film is determined by the three-axis refractive indexes and orientation state of the cellulose triacetate, as well as the refractive index, amount to be added, and orientation state of the disk-like compound serving as an additive. Although other additives, such as a plasticizer, slightly affect the development of retardation, their effects are generally small and can be therefore omitted.
Cellulose triacetate is a raw material that can be oriented with difficulty, usually, and it is therefore difficult to increase the ratio of orientation. It is therefore difficult to accomplish a large retardation value. In the case of attaining high retardation by using cellulose triacetate in the manner as in European Patent Application Publication No. 0911656A2, the additives largely contribute to the developed retardation.
The Re retardation value and the Rth retardation value are both defined as refractive indexes in three-axis directions, and therefore the ratio Re/Rth is mostly determined by an additive that largely contributes to the development of retardation. By investigating the Re/Rth ratio vs the orientation ratio, it was found that both are proportional to each other, and the Re/Rth ratio is increased as the orienting ratio is increased. In the case of varying the amount to be added, the same proportional relation is also obtained: The Re/Rth ratio is increased as the amount to be added is increased. The gradient of the Re/Rth ratio to the orienting ratio is determined by the raw material to be added. It was found that, in the case of the disk-like compound described specifically in the European Patent Application Publication No. 0911656A2, the gradient is small.
The orientation states of cellulose triacetate and additives, which orientation determines the retardation value, differ depending on the orienting method. Generally, a roll orienting method and a tenter orienting method are known, as examples of a monoaxially orienting method. In the former method, the width of a film is shrunk, making ny small, and therefore the (nx−ny) value is easily made larger, so that Re tends to be developed. In the latter method, a film is oriented in the transverse direction in the condition that the carrying direction is limited, and the (nx−ny) value is therefore scarcely made large. Therefore, the Re/Rth ratio to the orienting ratio is smaller in the latter case than in the former case.
Because the tenter orienting method has a tendency to decrease the dispersions of film thickness and optical performances, it is suitable as a method of producing an optical compensation sheet for a liquid crystal display device. When this method is applied to the aforementioned disclosed example, the increment of the Re/Rth ratio is about 0.01 or less, per 1% of the orienting ratio. When the target value of Re is close to the target value of Rth and the Re/Rth ratio is about 0.5, the orienting ratio must be 50% or more. It is difficult to actually attain this orienting ratio stably in the case of cellulose triacetate film resistant to orienting.
Also, as to the amount to be added, it is difficult to attain the optical performances to be intended by a possible increase in the amount to be added.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.